Carbide is a powder metallurgy product which is sintered in a vacuum furnace or a hydrogen reduction furnace with tungsten carbide powder as main component and cobalt or nickel as binder.
There is a shortage of cobalt resources in our country, and a large quantity of cobalt need to be imported every year. Although tungsten resources are abundant, with the significant increase in production in recent years, reserves and exploitation capacities thereof are reducing. Waste carbide contains up to 40% to 95% of tungsten, much higher than that in APT as raw material for production of carbide, and is of a very high value in use. Therefore, recycling of waste carbide is of great significance to rationally use and protection of existing resources and improvement in resource utilization. Currently, the technologies for recycling of waste carbide include: acid leaching method[1], zinc melting method[2], mechanical crushing method[3] and selective electrochemical dissolution method[4].
Acid leaching recycling process is relatively simple, but NO and SO2 gases discharged during the reaction process, causing serious harm to the environment, and the equipment requires corrosion protection, and safety operation should be guaranteed. Zinc melting method is widely used, but the method has some disadvantages, such as zinc residue, high energy consumption, complex equipment, etc. Mechanical crushing method requires strong crushing and abrasive equipment in practice because it is difficult to break carbide scrap. Furthermore, the oxidation of materials during crushing and milling process could easily lead to changes in mix ingredients, thus it is difficult to recycle high-quality alloy. In selective electrochemical dissolution process, waste carbide, which is used as anode, is put into an electrolysis cell with acid as electrolyte for electrolysis. Cobalt in the alloy becomes cobalt ions and enters the solution, and tungsten carbide which has lost cobalt as cohesive metal becomes loose alloy. Then cobalt powder can be prepared by precipitating cobalt-containing solution with ammonium oxalate, then calcining and reducing the resultant precipitates. Tungsten carbide can be used in carbide production after appropriate treatment, such as ball milling and breaking. Recovery of waste carbide by electrochemical dissolution process is simple in technology, but there will be an anode passivation, which makes the current efficiency greatly reduced, and the subsequent processing of the waste liquor generated during the electrolysis increases recovery cost.
Molten salt electrolysis process obtains the pure metal or alloy product of tungsten on the working electrode in the electrolyte of molten salt by electrochemical method. In the trend that short process, low cost and friendly to environment are required in the development of the metallurgical industry, molten salt electrolysis process is very interesting because of its unique advantages in respect of the manufacture of metals and their alloys, for example, small footprint of equipment, simple operation process and minor side effects to the environment, etc.
Liu[5] adopted the Na2WO4—ZnO—WO3 system to prepare tungsten coating by molten salt electrolysis using tungsten plate as anode. The particle size of the resulting product is about 3 μm, and zinc is also easily deposited when tungsten is deposited, rendering the product impure. Erdo{hacek over (g)}an[6] prepared tungsten powder by electrolysis reduction in CaCl2—NaCl molten salt system under argon atmosphere, using graphite rod and CaWO4 as anode and cathode respectively, with the particle size of obtained tungsten powder approaching 100 nm. Wang[7] prepared nanometer tungsten powder in NaCl—KCl molten salt system under argon atmosphere, using graphite rod and WS2 block as anode and cathode respectively, with the particle size of the product of 50-100 nm and the current efficiency of 94%. Wang, et al. adopted the CaCl2—NaCl—Na2WO4 system to directly prepare tungsten powder by fusion electrolysis using graphite rod as anode. Although the traditional tungsten-manufacturing process was shortened, the particle size of the resulting tungsten powder is relatively large with an average particle size of about 2 μm, which does not meet the nanoscale. Moreover, some impurities such as C, WC, W2C and the like appeared in the cathode product, and it is difficult to separate them by subsequent process.
From the above findings, most of the studies relating to preparing nanometer tungsten powders by molten salt electrolysis focused on the electrolysis of tungsten-containing actives. Compared to using tungsten-containing actives to prepare tungsten powder, electrolyzing waste carbide using molten salt to prepare nanometer tungsten powder has lower material cost; on the other hand, the key technology for this lies in the dissolution of tungsten in anode carbide, and effective isolation of tungsten from activated carbon atoms during electrolysis process.
Currently the prior technology for recycling waste carbide has shortcomings, such as long production process, large energy consumption, unfriendly to environment, product defects, etc. Therefore, it is very necessary to find out a recycling technology with short process, high efficiency and quality for recovering waste carbide. The method with waste carbide directly as anode and adopting molten salt electrolysis to obtain nanometer tungsten powder recovered on the cathode has not yet been reported. This method may greatly shorten the existing waste carbide recycling process without waste emission, and it is friendly to environment and has low energy consumption. Furthermore, the recycled tungsten powder may have a particle size of nanoscale.